1. Field of the Invention
This application claims the benefit of People's Republic of China. Application No 201010103958.5, filed on Jan. 21, 2010.
The present invention generally relates to a method for scanning a projective capacitive touch panel.
2. Description of the Related Art
Capacitive touch panels are divided into projective capacitive touch panels and surface capacitive touch panels. Different touch panels adopt different principles according to different configurations. The projective capacitive touch panel is disposed on a substrate with the other surface of the projective capacitive touch panel attached to the substrate. A controller electrically connects to the projective capacitive touch panel for driving the projective capacitive touch panel.
The projective capacitive touch panel includes two layers of conductive electrodes orthogonally placed. One layer of conductive electrodes includes first-axis electrodes parallelly arranged along a first-axis (electrode X). The other layer of conductive electrodes includes second-axis electrodes parallelly arranged along a second-axis (electrode Y). The two layers of orthogonally placed and mutually insulated conductive electrodes form an electrode matrix having intersections. The controller includes a pulse generator connected to electrodes Y and an integrator connected to electrodes X.
Conventionally, mutual capacitance are scanned by first, having the pulse generator generate a continuous low frequency pulse excitation signal to charge electrodes Y then, the integrator collects electric charges induced in a plurality of electrodes X at the intersections between the electrodes Y and the electrodes X. According to the electric charges, the controller calculates reference voltages of capacitances at the above mentioned intersections. Then other electrodes Y are charged by the pulse generator one by one, and the reference voltages of the capacitances at all the intersections between the electrodes Y and the electrodes X are calculated by the above mentioned method.
When the touch panel is touched, all of the mutual capacitance, capacitance at the intersection of two electrodes (herein referred to as mutual capacitance), are scanned by applying a continuous low frequency pulse excitation signal to obtain current voltages (voltages occurring in the present time) of each mutual capacitance at each intersection by the method mentioned above, then each of the current voltages is compared with the corresponding reference voltage. If a current voltage exceeds the corresponding reference voltage by a threshold value at an intersection, then the intersection is considered located in a touched area, and a touched point will be determined by calculating the centroid of the intersections that are located in the touched area.
The conventional scanning method scans all the mutual capacitance by applying a constant frequency excitation signal, which will cause some problems. The distance from each mutual capacitance to the integrator varies, that is, the span of one sensing electrode from the integrator to a specific intersection may be, different from the span of another sensing electrode from the integrator to another specific intersection, which means the resistance of a specific length of sensing electrode from an intersection to the integrator may be different from that of another length of sensing electrode. Meanwhile, the capacitance at each intersection is basically the same, thus the RC constant (a product of a resistance and a capacitance) of each mutual capacitance may be different accordingly. The RC constant is smaller as the mutual capacitance is nearer to the integrator, and the RC constant is greater as the mutual capacitance is farther to the integrator. For a large size touch panel, the RC constant of the mutual capacitance formed at the first electrode Y is much different from the RC constant of the mutual capacitance formed at the last electrode Y. If all of the mutual capacitances are scanned by a constant relative high frequency excitation signal, the current voltage will be very different between the mutual capacitance having different RC constant, which will affect the scanning accuracy. In order to guarantee the scanning accuracy, the frequency of the excitation signal is lowered to reduce the difference, but low scanning frequency means low scanning, speed and long scanning time.
Thus, it is desired to provide a method for scanning a projective capacitive touch panel that overcomes the above drawbacks of the conventional scanning method.